Battle of the Field of Celebrant
In 2510 ThA, the Balchoth, a people from east of Mirkwood and under the influence of DolGuldur, invaded Gondor. They crossed the Auduin near Gladden and swept away the defenders. Gondor tried to respond with forces from the south, but the were outnumbered and cut off, backed up to the the Limlight. There they meet Orcs form the MistyMountains.
Suddenly aid unlooked for came out of the north. Eorl and the Éothéod came and routed the Easterlings.
As a reward for their aid Cirion of Gondor gave the to Eorl and the Éothéod the land of Calenardhon to have as their kingdom.
This was accomplished with the Oaths of Eorl and Cirion